42 BASIS OF THE PROBLEM 



unfertilized is small, the number of the active spermatozoa being 

 so many times greater than the number of eggs. This simple 

 form of fertilization is characteristic of most plants and of many 

 animals. In plants there are certain complications, in particular 

 those connected with fertihzation with the help of insects, into 

 which it is not necessary to go. 



The increasing complexity of animal structure is only roughly 

 correlated with increasing complexity in the processes connected 

 with fertihzation. Though the most complex form of the process 

 is found among the highest animals, yet some animals, which 

 stand relatively high in the scale, exhibit a simple method of 

 fertilization. Of this the fish are an example, and, further, the more 

 highly developed or bony fish exhibit a simpler form than some 

 of the more lowly-organized cartilaginous fish. In fact, the stage 

 next above that of which the sea-urchin was given as an example 

 can be illustrated from the bony fish. The process in this group 

 consists in the approach of the male to the female alongside of 

 whom he swims, led by a rudimentary development of the sexual 

 instinct. "When the female ejects her gametes into the water, 

 the male does the same and the vastly greater number of the 

 male gametes ensures that all or nearly all of the eggs are 

 penetrated by a spermatozoon.^ 



The next stage is that characterized by external copulation. 

 The male clasps the female and when the latter ejects her eggs, 

 he extrudes his spermatozoa at the same time. The meeting of 

 the gametes thus still takes place outside the body of the female. 

 This form of fertilization is found among the Amphibia ; the 

 male frog, for example, has specially developed pads on his front 

 feet with which he embraces the female. Internal copulation, 

 though it is to be regarded as the most complex form of the 

 process, is found among such low forms as flat worms,^ of which 

 the common tapeworm is a member, among many higher 

 V Invertebrates such as snails and insects, as well as among 

 cartilaginous fish, birds, and mammals. The essential feature 

 of internal copulation is that the male is provided with a special 

 copulatory organ or penis which he inserts into the female. The 

 male gametes pass through the penis, which takes the form of 



1 A few bony fish, e. g. Anableps, copulate internally. "^ Internal copulation 



among the flat worms is of a very primitive type — the penis apparently, at least 

 at times, penetrating the female at any point. 



